1. Field of the Invention
The present invention relates to a time-division transmission technique for use in, for example, digital mobile radio systems and, more particularly, to a transmission technique which uses a modified full-duplex arrangement where two channels are assigned to each base-mobile link. Simultaneous two-way transmission is provided by transmitting sequential bursts from the base and mobile stations which are alternated between the first and second frequencies of the two channels, with each station using a different one of the channels at any instant of time.
2. Description of the Prior Art
In mobile radio systems using time-division retransmission (TDR), the base-to-mobile and mobile-to-base links share the same frequency channel in a time-division manner. As shown, for example, in U.S. Pat. No. 4,383,332 issued to B. Glance et al on May 10, 1983, because of channel reciprocity, the diversity processing for both directions of transmission can be done at the base station alone, thus permitting the use of a single antenna and simplified circuitry at the mobile. In such TDR systems, however, it was found that the influence of the rms delay spread (.tau..sub.o) of the multipath medium provided a sharp decline in detection performance where the delay spread is greater than 25% of the symbol period, T. More specifically, the delay spread is defined as the spread in arrival times at the receiver of a symbol burst in travelling the multipath routes between the transmitter and receiver.
For TDR using 32-kb/s encoding and 4-PSK modulation, T is approximately 28 microseconds, so that frequency selectivity becomes important if .tau..sub.o &gt;7 microseconds. For 2-PSK modulation, this threshold .tau..sub.o would be 3.5 microseconds. The condition where the delay spread is greater than 25% of the symbol period is found to occur often enough and with sufficient repeatability to be regarded seriously. One possible conclusion is that system design objectives should emphasize long symbol periods, T.
The problem remaining in the prior art is to provide a transmission technique which will permit longer symbol periods, to lower the relative rms delay spread (J/T) while avoiding reduced system capacity.